The microphthalmia transcription factor (MITF) is required for terminal differentiation of developmentally unrelated cell types including osteoclasts and melanocytes. MITF accomplishes this by regulating distinct sets of target genes in distinct cell types. We are trying to define the molecular mechanisms that account for the ability of MITF to regulate target genes in osteoclasts. Our recent studies provide two mechanisms that can at least partially account for the ability of MITF to selectively regulate target genes in osteoclasts. First, MITF interaction with the Ets-family factor PU.1 is required for regulation of target genes and for differentiation of fully functional osteoclasts. Second, MITF is a direct target for Receptor Activator of NF-kappaB Ligand (RANKL) action through a p38 Mitogen Activated Protein Kinase (MAPK) signaling pathway. Our overall hypothesis is that modification of MITF, through both its' interaction with PU.1 and through phosphorylation by the RANKL/p38 MAPK, leads to assembly of a unique complex of proteins capable of directing an osteoclast-specific gene expression program. The proposal has two specific aims: Aim 1. To determine the mechanism that underlies MITF and PU.1 co-operation in osteoclast target gene activation and differentiation; Aim 2. To determine the mechanism by which the p38 Mitogen Activated Protein Kinase pathway increases MITF activity in osteoclasts. Understanding the mechanism of MITF action in osteoclasts will provide a basic understanding of osteoclast gene expression and differentiation, and may also provide new molecular targets that can be used to disarm the osteoclast in human diseases that result at least in part from the hyperactivity of this cell type.